The present invention relates to a variable valve lift system. More particularly, the present invention relates to a variable valve lift system in which a valve lift is varied in two stages through changes in valve lift and cam duration by selectively rigidly engaging a tappet shim.
An engine is typically structured such that cams mounted on a camshaft open and close intake and exhaust valves by rotation of the camshaft. In some modem engines, a variable valve control system is used in which valve timing and the amount of lift of the valves are controlled to improve performance and reduce fuel consumption.
With reference to FIGS. 1 and 2, a conventional variable valve lift system for controlling the degree of valve lift includes a tappet shim 3 that receives operational force from cam 1, and a body 7 for transmitting an operational force from the tappet shim 3 to a valve 5. Supports 11 are mounted to four points on the tappet shim 3 and supports 13 are mounted to four points on the body 7. The four points of the tappet shim 3 oppose the four points of the body 7. An elastic member 9 is interposed between each opposing pair of supports 11 and 13 to provide a biasing force to the supports 11 in the direction of the tappet shim 3 and a biasing force to the supports 13 in the direction of the body 7.
Further, stoppers 17 and 19 are mounted in hydraulic pressure chambers 21 and 23, respectively, of the body 7. An elastic member 15 is interposed between the stoppers 17 and 19. The stoppers 17 and 19 move by interaction with the elastic member 15 and with oil in the hydraulic pressure chambers 21 and 23. Oil holes 25 lead into the hydraulic pressure chambers 21 and 23 to enable the supply of oil thereto.
In the variable valve lift system structured as in the above, if oil is not supplied through the oil holes 25, the stoppers 17 and 19 are positioned within the hydraulic pressure chambers 21 and 23, respectively, as a result of receiving a biasing force of the elastic member 15. As a result, a state in which the tappet shim 3 is not upwardly supported is maintained. Hence, the operational force of the cam 1, transmitted to the tappet shim 3, is absorbed by the elastic members 9 positioned between the supports 11 and 13 such that the valve 5 is operated through the body 7. With such operation, the amount of lift of the valve 5 cannot be varied.
On the other hand, with reference to FIGS. 3 and 4, if oil (a) is supplied through the oil holes 25, the stoppers 17 and 19 overcome the biasing force of the elastic member 15 such that the stoppers 17 and 19 are forced in a direction toward each other to be positioned at the outside edge of the hydraulic pressure chambers 21 and 23, respectively. As a result, the tappet shim 3 is upwardly supported, that is, the tappet shim 3 cannot be downwardly displaced into the body 7 in this state. Hence, the operational force of the cam 1, transmitted through the tappet shim 3, operates the valve 5 through the body 7 after it is transmitted by the supports 11 and 13 and the stoppers 17 and 19. The amount of lift of the valve 5 is increased as a result.
However, in the above variable lift system, since the two stoppers 17 and 19 support the tappet shim 3 during application of a load by a cam lobe, the stoppers 17 and 19 experience wear. Further, the elastic member 15, interposed between the stoppers 17 and 19, does not precisely return the stoppers 17 and 19 when oil is exhausted from the hydraulic chambers 21 and 23.
The present invention provides a variable valve lift system in which valve lift is varied in two stages through changes in valve lift and cam duration by selectively rigidly engaging a tappet shim, and in which the durability of the support and the return of the stopper are improved. In a preferred embodiment, a tappet shim receives an operational force from the cam. A body transmits an operational force of the tappet shim to the valve. First and second supports, the first supports being provided on a surface of the tappet shim opposing the body and the second supports being provided on a surface of the body opposing the tappet shim, are supported by elastic members. First and second stoppers, mounted respectively in first and second hydraulic pressure chambers, formed on opposite sides of the body, are provided with an elastic member interposed between the first stopper and the body and an elastic member interposed between the second stopper and the body. The first and second stoppers selectively rigidly support the tappet shim in a second lift stage. A third stopper is mounted in a third hydraulic pressure chamber, which is formed in the body at a location opposing the tappet shim, and an elastic member is interposed between the third stopper and the body.
Preferably, the elastic members positioned in the first, second, and third hydraulic pressure chambers provide a biasing, pulling force to create resistance to hydraulic pressure acting on the first, second, and third stoppers. It is also preferable that the third stopper be mounted at substantially a center portion of the body to support the tappet shim in a state that is balanced during operation under the lobe load of the cam. It is further preferable that the third stopper is further configured to provide a higher position than the first and second stoppers during operation such that the third stopper supports the tappet shim at a high lift of the valve when oil is supplied to the third hydraulic pressure chamber.